1. Field of the Invention
Aspects of the present invention are directed to surface mounted interconnections and, more particularly, to surface mounted interconnections that minimize dimensional instability and solder stress.
2. Description of the Background
Generally, conventional dual in-line memory module (DIMM) interconnections employing surface mounted technology (SMT) have a connector housing (connector) molded with liquid crystal polymers (LCPs). In these interconnections, contacts are inserted and/or stitched into the connector and subsequently retained by the connector geometry before being simultaneously soldered to a print circuit board (PCB) to which the DIMM is connected.
However, it has been observed that molded LCPs are dimensionally unstable in the directions of the thermal expansion and contraction of the PCB. In detail, the soldering operation heats the PCB and the connector. Subsequently, since the PCB and the connector have mismatched coefficients of thermal expansion (CTEs), the PCB and the connector experience mismatched thermal expansions during the reflow operation, and then mismatched contraction when the PCB and the connector cool. The critical period is when the assembled connector system cools through the solder solidus, when the connector-to-PCB interface is locked into position. Further cooling to room temperature at different rates causes stresses to build up at the solder joint. In addition, it has been observed that molded LCPs also demonstrate final dimensional instability. After the expansion and contraction during the heating and cooling cycle, the connectors have demonstrated a permanent size change. This also contributes to increased stress at the solder joint.
It has further been observed that the resulting mismatched thermal expansions and contractions generate dimensional instability and stresses between the PCB and the contacts of the connector. Such instabilities and stresses may be manifested in a warped and/or similarly affected PCB and/or an altered position of mating contacts within the connector that increase manufacturing costs and risks of failure of the conventional interconnections at both the separable interface and the connector-to-PCB interface.
One solution to these problems has been to anneal the connector. However, it has been seen that this solution has its own drawbacks. Namely, annealing can cause the connector to twist or bow. Further, if a connector's contacts are stitched prior to the annealing process the high annealing temperature can cause base metal migration in the contacts and leads. Base metal migration interferes with the overall contact resistance of the interface in that it introduces the likelihood of oxide formation (e.g., Nickel oxide, copper oxide) over the gold mating interface, increasing contact resistance, and decreasing contact integrity